The present invention relates generally to the art of aligning co-rotatable in-line machine shafts which are coupled together for operation by means of a shaft coupling. More particularly, the invention relates to methods and systems for acquisition of data from which the amount of misalignment can be determined, and from which machine moves to bring the shafts into alignment can be determined.
As is well known, whenever two rotating machine shafts are coupled together, such as the shaft of an electric motor and the shaft of a pump, it is important that the shafts be aligned within predetermined tolerances. Such shafts, when in perfect alignment, have their extended center lines (axes of rotation) coinciding along a straight line. Misalignment can lead to vibration, excessive wear, and ultimate destruction of couplings, bearings, seals, gears and other components.
There are two relevant misalignment components, and either or both may be present in a given situation. One misalignment component is offset misalignment, also termed parallel misalignment or simply offset. In the case of offset misalignment, shaft center lines may be parallel, but they do not intersect. The other misalignment component is angular misalignment, and occurs when shafts intersect at an angle. Angular misalignment is also termed angularity, and is manifested as a difference in distance between coupling hub faces across a diameter of the coupling hub faces.
A number of shaft alignment methods are known, which generally have in common the use of suitable alignment fixtures, also termed alignment brackets. The alignment brackets are employed to measure particular relative displacements (also termed offsets) as the shafts are rotated together through one revolution, while stopping at 0.degree., 90.degree., 180.degree. and 270.degree. angular rotation positions to take readings. Each relative displacement is measured between a point referenced to one of the shafts by means of the alignment bracket and a point on the other shaft. Dial indicators are often employed, these dial indicators having a plunger which moves a hand on the face of the dial indicator.
The readings are then used to calculate machine moves which will bring the shafts into alignment. The 0.degree., 90.degree., 180.degree. and 270.degree. angular positions at which readings are conventionally taken lie in geometric planes in which either of the machines, for example the motor, may be moved for purposes of alignment. In particular, the mounting bolts of the machine may be loosened and the machine may be either moved in a horizontal plane; or the machine may be moved in a vertical plane by placing or removing shims under one or more of the feet of the machine, or both. There are well developed calculation methods and procedures known in the art for determining what machine moves to make to achieve an aligned condition based on measurement of relative displacement (offset) data at the 0.degree., 90.degree., 180.degree. and 270.degree. positions mentioned, which may be termed calculation angular positions.
Although mechanical dial indicators are referred to above, it will be appreciated that other forms of measurement devices may be employed, including various optical and mechanical transducers. Also, although it is relative displacement which is actually determined, it will further be appreciated that absolute readings may be taken, referenced to a particular angular position, and a simple subtraction operation performed to determine relative displacement.
There are various points where relative displacements may be measured, depending upon the particular alignment geometry employed. However, a commonly employed method is the reverse indicator method wherein a pair of relative displacements in a radial direction are measured at each of the calculation angular positions.
As usually practiced, the reverse indicator method employs either one or two alignment brackets. An alignment bracket has a base firmly clamped or otherwise affixed to one shaft, and an extension bar or arm extends laterally from the base in a direction generally parallel to the shafts across the coupling over to a reference point adjacent a point on the periphery of the other shaft. A device for measuring displacement, such as a dial indicator, is positioned so as to measure relative displacement in a radial direction (offset) from the reference point to the point on the periphery of the other shaft as the shafts are rotated together while stopping at the 0.degree., 90.degree., 180.degree. and 270.degree. angular positions to take and record readings. The position of the alignment bracket is then reversed so as to be fixedly referenced to the other shaft, establishing a reference point adjacent a point on the periphery of the one shaft, and the procedure is repeated. Alternatively, a pair of alignment brackets may be employed for simultaneous readings.
From the geometry just described, it will be appreciated that the reference point on the alignment bracket attached to the one shaft rotates about the projected centerline (axis of rotation) of the one shaft to define a circle centered on that projected centerline, and vice versa for the other shaft, and that the distance and direction of the distance between the two shaft centerlines as projected can be determined at any transverse plane along the shaft axes. From the thus measured distances and directions of the distances between the two shaft centerlines as projected in two transverse planes, both the offset misalignment component and the angular misalignment component may be calculated.
Another method which is sometimes employed is known as the face-and-rim method. The "rim" part of this method is measurement of a relative displacement in a radial direction as just described, and the "face" part of this method is measurement of a relative displacement in an axial direction, again at each of the predetermined angular positions which lie in geometric planes in which either of the machines connected to the shafts may be moved in order to achieve an aligned condition. Typically, but not necessarily, the "face" and "rim" measurements are taken generally in the same transverse plane along the shaft axes. While the face-and-rim method thus directly measures angular misalignment, it nevertheless is generally considered to be less accurate than the reverse indicator method.
The techniques of the present invention are applicable to either the reverse indicator method or the face-and-rim method, as well as the other related methods where relative displacement measurements are made at a plurality of angular positions, and are then used in subsequent calculations, particularly to determine machine moves for alignment purposes.